Linear accelerations are often detected with the assistance of Z sensors which are based on the use of seismic masses which are deflectably fixed and asymmetrically positioned with respect to a torsion spring (See European Patent No. 0773443 A1).
In Z sensors, the seismic mass is usually composed of a rocker structure exposed by trench etching techniques, referred to below as a rocker, which on account of the asymmetrical mass distribution of the rocker is twisted and deflected about a rotational axis, defined by the torsion spring, during an acceleration in the Z direction. Beneath the rocker which has been exposed by etching are situated electrodes which are formed by a thin conductive layer on the substrate and which together with the seismic mass represent a differential capacitor. Mechanical deflections of the rocker result in changes in capacitance of the individual capacitor regions, and may be detected as differential signals and further processed by an evaluation circuit.
For acceleration sensors it is common to check the mobility of the seismic mass and the functionality of the sensor, using a test signal, and optionally to perform calibration. Application of a test voltage to the capacitor electrodes generates an electrostatic force which results in a deflection of the seismic mass which is equivalent to the acceleration.
During processing, as the result of the technology used, charges are generated on surfaces exposed by etching. These charges are sometimes localized, and also remain in the region between the seismic mass and parallel electrode surfaces, and thus generate a permanent electrostatic force which results in faulty deflection of the seismic mass, and also results in a sensor signal, even without application of an acceleration. This “zero-point signal,” also referred to as offset of the output signal, is usually compensated for in the evaluation circuit.
However, a problem arises when the charge density on relevant surfaces changes. This may be caused, for example, by high temperature, for example in the range above 100° C., or by aging processes. In such cases surface charge drift may result, which is directly associated with offset drift. Permanent monitoring and compensation would be costly and complicated. To reduce the influence of drifting surface charges on the output signal of lateral acceleration sensors, so-called X sensors, a clocked voltage is applied according to an intelligent clocking scheme to the electrodes which form the capacitor (See German Patent Application No. DE 103 50 536 B3).
However, this method requires substantial symmetry of the impinged regions as a basic geometric condition, with the result that Z sensors having an asymmetrical rocker may be used only in the region of the electrodes which are part of the differential capacitor.
However, on account of the considerable leverage it is specifically the outer surface region, which provides the asymmetry of the rocker, which greatly contributes to tipping of the rocker. If the density of the surface charge changes in this surface region, this has a particularly strong effect on the drift of the output signal, i.e., the test signal response.